Codec compensation techniques for channel analysis applications

ABSTRACT

Methods and apparatus to perform and/or analyze communication channels are described. In one embodiment, a test signal may be generated and transferred using a first signal route to a line-under-test in response to line qualification mode. The first signal route may include a compensation device to modify the test signal. In some embodiments, a reflected signal from the line-under-test may be received in response to the test signal. A second signal route may be utilized to route the reflected signal in response to line qualification mode.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present application is related to U.S. patent application Ser. No.10/457,092, filed Jun. 6, 2003.

FIELD

The subject matter disclosed herein generally relates to techniques totest signal propagation media.

DESCRIPTION OF RELATED ART

Line qualification can determine whether a signal propagation medium iscapable of providing communications in accordance with a particularcommunications standard. For example, line qualification can beperformed to determine whether a signal propagation medium can be usedwith DSL standards and variations thereof (including but not limited toADSL, SHDSL, and VDSL) (DSL standards and variations thereof hereafterare referred to as xDSL). For a description of xDSL standards, see, forexample, ITU-T G.991.1, High bit rate Digital Subscriber Line Ttransceivers (1998); ITU-T G.991.2, Single-pair high-speed DigitalSubscriber Line transceivers (2001); ITU-T G.992.1 ADSL standard G.dmt(1999); and related standards. Current DSL and ADSL line qualificationtechniques typically use dedicated and very expensive hardware and caninvolve an expensive procedure of dispatching of a trained technician tothe client modem site. Currently, it is impractical to build linequalification systems inside cost-sensitive DSL and ADSL mass productionmodems.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with features and advantages thereof, may best be understood byreference to the following detailed description when read with theaccompanying drawings in which:

FIG. 1 depicts in block diagram format a communications system inaccordance with an embodiment of the present invention;

FIG. 2 depicts one possible implementation of a modem in accordance withan embodiment of the present invention; and

FIG. 3 depicts an example of a compensation device in accordance with anembodiment of the present invention.

Note that use of the same reference numbers in different figuresindicates the same or like elements.

DETAILED DESCRIPTION

FIG. 1 depicts in block diagram format a communications system 5. Modem10 may provide communications between a personal computer (PC) 20 and acentral office modem 30 using a communications medium such as a twistedpair telephone line. Modem 10 may provide communications capabilities inaccordance, for example, with xDSL and/or other protocols.

Modem 10 may communicate with PC 20 using a cable or bus compliant, forexample, with Peripheral Component Interconnect (PCI), Universal SerialBus (USB), Ethernet (e.g., IEEE 802.3), and/or IEEE 1394, although othertechniques may be used such as wireless techniques described, forexample, in IEEE 802.11 (and related standards).

In accordance with an embodiment of the present invention, a modem mayperform line qualification of a line, such as one or a combination of atwisted pair telephone line, coaxial cable, or other signal propagationmedium, to determine whether the line is capable of providing xDSL orother communications services. The modem may at least perform linequalification of lines of different lengths and having one or morebridge taps. For example, a bridge tap may represent a juncture in whichanother modem or device may access the line. To perform linequalification, the modem may transmit test signals to the line andprocess signals reflected by the line in response to the test signals.

FIG. 2 depicts one possible implementation of a modem 200 in accordancewith an embodiment of the present invention, although otherimplementations may be used. One embodiment of modem 200 may includesignal processor 230, interface 240, transmitter 210, compensationdevice 215, receiver 220, line driver 245, hybrid 260, and switches252A-252D and 254A-254C, although other implementations may be used.Modem 200 may be implemented as any or a combination of: hardwiredlogic, software stored by a memory device and executed by amicroprocessor, firmware, an application specific integrated circuit(ASIC), and/or a field programmable gate array (FPGA). In oneimplementation, one advantage, although not a necessary feature, is thatline qualification capabilities may be manufactured using cost-effectiveanalog front end devices.

Modem 200 may communicate with a far end modem or other device usingline 250. Modem 200 may operate in line qualification mode and modemmode. In line qualification mode, modem 200 may operate in “transmit”and “receive” modes. In “transmit” and “receive” line qualificationmodes, switches 252A-252D may be set to close (i.e., transfer signals)and switches 254A-254C may be set to open (i.e., to not transfersignals). During “transmit” line qualification mode, test signalstransmitted by signal processor 230 to line 250 may traverse interface240, transmitter 210, closed switch 252A, compensation device 215,closed switch 252B, line driver 245, and closed switch 252C. Such testsignals may bypass hybrid 260 and may avoid introducing board echoattributable to the hybrid 260.

In “receive” line qualification mode, a reflection signal from line 250based on a test signal transmitted in “transmit” line qualification modemay be transferred by closed switch 252D to amplifier 227. Accordingly,modem 200 may route reflection signals during “receive” linequalification mode to bypass the hybrid 260 to avoid introducing boardecho attributable to the hybrid 260.

In one implementation, if line qualification passes, then modem 200 mayoperate in normal modem mode. When the modem 200 operates in modem mode,switches 254A-254C may be set to close (i.e., transfer signals) andswitches 252A-252D may be set to open (i.e., not transfer signals). Totransmit signals in modem mode, transmitter 210 may transmit signals toline 250 through closed switch 254A, line driver 245, closed switch254B, and hybrid 260. To receive signals in modem mode, amplifier 227 ofreceiver 220 may receive signals from line 250 through hybrid 260 andclosed switch 254C. Additional description of receiving signals duringmodem mode is provided with respect to description of receiver 220. Inone implementation, a controller (such as signal processor 230) maycontrol the open/close states of switches 252A-252D and 254A-254C duringline qualification and modem modes.

Signal processor 230 may perform modulation/demodulation,encoding/decoding of signals in accordance with xDSL. For example,signal processor 230 may operate in compliance with xDSL standards suchas ITU-T G.991.1, ITU-T G.991.2, and ITU-T G.992.1. Signal processor 230may determine and indicate whether line 250 passes or fails linequalification for xDSL or another service. For example, signal processor230 may control the amplitude and duration of test signal pulses. Basedon signals reflected by the line 250 in response to the test signals,signal processor 230 may modify the duration and/or amplitude of thetest signals. Based on signals reflected by line 250 in response to testsignals, signal processor 230 may determine characteristics of the line(such as the length of the line, whether a bridge tap exists in theline, and cross talk noise level) and/or whether the line passes linequalification for xDSL or other services.

For example, signal processor 230 may use time domain reflectometry(TDR) techniques to determine the length of line 250. The length of theline may be an important factor in whether the line passes linequalification for xDSL service although other factors may be consideredsuch as the existence and distance of any bridge taps in the line andcross talk noise level. Signal processor 230 may be implemented as anyor a combination of: hardwired logic, software stored by a memory deviceand executed by a microprocessor (for example, software executed forexample by PC 20 or a central processing unit of modem 200 (notdepicted)), firmware, an application specific integrated circuit (ASIC),and/or a field programmable gate array (FPGA).

Compensation device 215 may reduce distortions in test signals providedthrough transmitter 210. In one implementation, compensation device 215may provide a gain boost, high-pass filtering, and phase modification oftest signals. FIG. 3 depicts one possible implementation of compensationdevice 215, in accordance with an embodiment of the present invention.Compensation device 215 may include gain stage 310, filter 320, andphase modifier 330. Gain stage 310 may provide a power gain boost of thetest signal of approximately 50 dB. Filter 320 may high-pass filter thegain boosted test signal from gain stage 310. For example, filter 320may transfer the gain boosted test signal for frequencies aboveapproximately 138 kHz. Phase modifier 330 may provide aone-hundred-eighty (180) degree phase shift of the signal transferredfrom filter 320.

In order to estimate the length of line 250, the attenuated amplitude ofthe reflected signal must be reliably detected so that the time intervalbetween the test signal and peak of the reflected signal can beaccurately calculated. In some implementations, low-pass filtering ofthe test signal (e.g., by LPF 234) may spread the width of the testsignal and generate ripples in the test signal that could bury thereflected signal thereby making TDR measurement impossible. Compensationdevice 215 may reduce the stop band signal attenuation caused bylow-pass filtering. The time interval between the compensated testsignal and loop echo may then be accurately calculated to accuratelydetermine the length of the line 250 or other characteristics of theline 250.

Interface 240 may transfer signals to transmitter 210 and/or receivesignals from receiver 220. For example, interface 240 may transfersignals to and from a signal processor 230. Interface 240 may transfersignals to and from a personal computer (PC) (not depicted).

Transmitter 210 may process signals for transmission to a far end modemor central office in compliance, for example, with xDSL standards,although other standards may be complied with. One implementation oftransmitter 210 may include digital-to-analog converter (DAC) 233, lowpass filter (LPF) 234, and line driver 235, although otherimplementations can be used. DAC 233 may receive signals from interface240. DAC 233 may be implemented as a conventional digital-to-analogconverter. LPF 234 may receive signals from DAC 233. LPF 234 may beimplemented as a filter having a pass band from approximately DC to 138kHz. Line driver 235 may receive signals from LPF 234. Line driver 235may provide a voltage gain of approximately 15.7 dB. In oneimplementation, a controller (such as signal processor 230) may programthe characteristics (e.g., gain, pass band, on/off state) of each of thecomponents of transmitter 210.

Receiver 220 may process received signals in compliance, for example,with xDSL standards, although other standards may be complied with.Received signals may be transmitted by a far end modem or central officevia a bridge tap or be a reflection of a transmitted test signal. Oneimplementation of receiver 220 may include amplifier 227, low passfilter (LPF) 228, and receiver analog-to-digital converter (ADC) 229,although other implementations may be used.

During “receive” modem mode, amplifier 227 may receive signals from line250 through hybrid 260 and closed switch 254C, but during “receive” linequalification mode, amplifier 227 may receive reflected signals fromline 250 through switch 252D. Amplifier 227 may provide a voltage gainin the range of approximately 0 to 9 dB. LPF 228 may be implemented as alow pass filter having a pass band of approximately DC to approximately552 or 1104 kHz. LPF 228 may provide signals to ADC 229. ADC 229 mayconvert signals from analog to digital formats. ADC 229 may providedigital format signals to interface 240.

MODIFICATIONS

The drawings and the forgoing description gave examples of the presentinvention. The scope of the present invention, however, is by no meanslimited by these specific examples. Numerous variations, whetherexplicitly given in the specification or not, such as differences instructure, dimension, and use of material, are possible. The scope ofthe invention is at least as broad as given by the following claims.

1. An apparatus comprising: a test signal generator to provide a testsignal and selectively transfer the test signal using a first signalroute to a line-under-test in response to line qualification mode,wherein the first signal route includes a compensation device to modifythe test signal; a receiver to receive a reflected signal from theline-under-test, wherein the reflected signal comprises a signalreflected by the line-under-test in response to the test signal, whereinthe receiver selectively uses a second signal route to route thereflected signal in response to line qualification mode; and a signalprocessor to modify one of a duration and amplitude of the test signalbased on the reflected signal, wherein the first signal route comprisesa route to bypass a hybrid line interface element and the second signalroute comprises a route to bypass the hybrid line interface element,wherein the first signal route and second signal route are to bypass thehybrid line interface to reduce introduction of board echo attributableto the hybrid line interface.
 2. The apparatus of claim 1, wherein thecompensation device is to reduce distortions in the test signal.
 3. Theapparatus of claim 1, wherein the compensation device is to reduce widthspreading and ripples in the test signal.
 4. The apparatus of claim 1,wherein the compensation device is to provide gain boost, high-passfiltering, and one-hundred-eighty (180) degree phase shift of the testsignal.
 5. The apparatus of claim 1, wherein the receiver is toselectively receive a received signal from the line-under-test using athird signal route in response to modem mode, and the third signal routeincludes processing by the second signal route and the hybrid lineinterface element.
 6. The apparatus of claim 1, wherein the test signalgenerator is to selectively control signal routes of the test signal andthe reflected signal in response to line qualification mode.
 7. Theapparatus of claim 1, wherein the test signal generator is toselectively modify the energy of the test signal based, in part, on thereflected signal.
 8. The apparatus of claim 1, wherein the test signalgenerator is to determine a length of the line-under-test based on thetest signals and reflected signals.
 9. The apparatus of claim 8, whereinthe test signal generator is to determine whether the line-under-testpasses line qualification, in part, on the length of theline-under-test.
 10. The apparatus of claim 1, further comprising afourth signal route to selectively transfer signals to theline-under-test in response to modem mode, wherein the fourth signalroute includes processing by a component not processed within the firstsignal route.
 11. The apparatus of claim 10, wherein the component notprocessed within the first signal route comprises a hybrid lineinterface element.
 12. The apparatus of claim 1, wherein the test signalgenerator is to selectively control signal routes of a signaltransmitted from the test signal generator and a received signal inresponse to modem mode.
 13. A method comprising: selectivelytransferring a test signal using a first signal route to aline-under-test in response to line qualification mode, wherein theusing the first signal route comprises reducing at least one distortionof the test signal; selectively transmitting signals to theline-under-test using a second signal route in response to modem mode,wherein the first signal route avoids processing of at least onecomponent of the second signal route; and modifying one of a durationand amplitude of the test signal based on the reflected signal, whereinthe first signal route comprises a route to bypass a hybrid lineinterface element and the second signal route comprises a route tobypass the hybrid line interface element, wherein the first signal routeand second signal route are to bypass the hybrid line interface toreduce introduction of board echo attributable to the hybrid lineinterface.
 14. The method of claim 13, further comprising: receiving areflected signal from the line-under-test, wherein the reflected signalcomprises a signal reflected by the line-under-test in response to thetest signal, wherein the receiving selectively uses a third signal routeto route the reflected signal in response to line qualification mode;and selectively receiving a received signal from the line-under-testusing a fourth signal route in response to modem mode, wherein thefourth signal route includes the third signal route and at least avoidedone component of the third signal route.
 15. The method of claim 13,wherein the at least one component of the second signal route comprisesa filter.
 16. The method of claim 13, further comprising selectivelymodifying the energy of the test signal based, in part, on the reflectedsignal.
 17. The method of claim 13, further comprising determining alength of the line-under-test based on the test signals and reflectedsignals.
 18. The method of claim 17, further comprising determiningwhether the line-under-test passes line qualification based, in part, onthe length of the line-under-test.
 19. The method of claim 13, whereinthe reducing at least one distortion of the test signal comprisesreducing width spreading and ripples in the test signal.
 20. The methodof claim 13, wherein the reducing at least one distortion of the testsignal comprises gain boosting, high-pass filtering, andone-hundred-eighty (180) degree phase shifting the test signal.
 21. Asystem comprising: a modem comprising: a test signal generator toprovide a test signal and selectively transfer the test signal using afirst signal route to a line-under-test in response to linequalification mode, wherein the first signal route includes acompensation device to modify the test signal, a receiver to receive areflected signal from the line-under-test, wherein the reflected signalcomprises a signal reflected by the line-under-test in response to thetest signal, wherein the receiver selectively uses a second signal routeto route the reflected signal in response to line qualification mode; asignal processor to modify one of a duration and amplitude of the testsignal based on the reflected signal; and an interface device toexchange signals with the modem, wherein the first signal routecomprises a route to bypass a hybrid line interface element and thesecond signal route comprises a route to bypass the hybrid lineinterface element, wherein the first signal route and second signalroute are to bypass the hybrid line interface to reduce introduction ofboard echo attributable to the hybrid line interface.
 22. The system ofclaim 21, wherein the interface device is compatible with PCI.
 23. Thesystem of claim 21, wherein the interface device is compatible with PCIexpress.
 24. The system of claim 21, wherein the interface device iscompatible with Ethernet.
 25. The system of claim 21, wherein theinterface device is compatible with Universal Serial Bus.
 26. The systemof claim 21, wherein the compensation device is to reduce distortions inthe test signal.